Stable dispersions of monocrystalline nanometric silver particles

ABSTRACT

A concentrated dispersion of nanometric silver particles, and a method of producing the dispersion, the dispersion including a first solvent; a plurality of nanometric silver particles, in which a majority are single-crystal silver particles, the plurality of nanometric silver particles having an average secondary particle size (d 50 ) within a range of 30 to 300 nanometers, the particles disposed within the solvent; and at least one dispersant; wherein a concentration of the silver particles within the dispersion is within a range of 30% to 75%, by weight, and wherein a concentration of the dispersant is within a range of 0.2% to 30% of the concentration of the silver particles, by weight.

This application is a continuation of U.S. Ser. No. 13/991,905, filedJul. 7, 2013, which is the national phase of PCT/US2011/063459, filedDec. 6, 2011, which claims priority from GB 1020556.5, filed Dec. 6,2010. The contents of the aforesaid applications are incorporated hereinby reference.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to stable, concentrated dispersions ofnanometric silver particles, and to a method of producing suchdispersions.

Nanometric silver particles are seeing increasing use in thepharmaceutical industry, especially in the field of wound management.Dispersions of fine silver particles are widely used in the manufactureof conductive inks and electrically conductive films for applicationssuch as internal electrodes in multi-layer capacitors, interconnectionsin multi-chip components, conductive lines in defoggers and deicers,photovoltaic modules, resistors, inductors, antennas, membrane switches,electromagnetic shielding, thermally conductive films, light reflectingfilms, and conducting adhesives. In many existing and emergingtechnologies, demand for ultrafine silver particles having specificproperties is increasing.

In some applications, required features of such particles may relate toat least one of: average particle size, narrow particle sizedistribution, particle density, and crystalline grain structure.

Nanometric silver particles have seen commercial use as dispersions inorganic solvents. The stability of such dispersions may typically beguaranteed for up to six months.

The production of fine silver particles by precipitation of theparticles from a liquid medium is well known. However, as taught by U.S.Pat. No. 6,277,169 to Hampden-Smith, et al., such liquid precipitationtechniques are often difficult to control to produce particles havingthe desired characteristics. Specifically, U.S. Pat. No. 6,277,169discloses that it is especially difficult by the liquid precipitationroute to obtain particles having dense, spherical particle morphologyand good crystallinity.

The production of small silver particles by reducing silver oxide (e.g.,using hydrogen peroxide) in aqueous liquid media is known. The sparinglysoluble silver oxide may be dissolved in the reaction media prior to thereduction reaction by complexing the silver ions with ammonia, asdisclosed by WO Patent Publication No. 2003/080231.

Various liquid precipitation techniques may promote agglomeration oraggregation of the silver particles, as well as particle sedimentation.Such agglomeration may be undesirable for many reasons, among them: theparticle size distribution and mean particle size may be deleteriouslyaffected, and contaminants in the mother liquor may be occluded betweenparticles, reducing product purity. We have found that the specificelectric resistivity of thin films formed from such contaminated silverparticles may disadvantageously be increased. Also agglomeratedparticles and/or sedimentation may clog the nozzles in ink jet printingheads, detracting from the robustness of the printing process.

Various liquid precipitation techniques may promote the formation ofpolycrystalline silver particles. We have found that suchpolycrystalline silver particles may disadvantageously exhibit a higherspecific electric resistivity. Moreover, they may display an appreciablyhigher propensity to agglomeration and to sedimentation.

The advances in the production of silver nanoparticles notwithstanding,the present inventors have recognized a need for improved silvernanoparticles and silver nanoparticle dispersions, and for methods ofproducing such nanoparticles and dispersions thereof.

SUMMARY OF THE INVENTION

We have found that chemical reduction in an aqueous medium, according tothe present invention, may have the greatest potential for industrialscale production of concentrated nanometric silver particle dispersions(of up to 75%, by weight) that may display superior stability (24 monthsor more) and may also enable the production of thin silver films havingextremely low specific electric resistivity (e.g., 2.5×10⁻⁶ ohm·cm orless). However, various additional processing steps may be required toensure that the nanometric silver particles produced, typically asdilute dispersions, do not undergo disadvantageous agglomeration andother transformations as the concentration of the dispersion isincreased, and as the washing and solvent displacement are effected.

According to teachings of the present invention there is provided aconcentrated dispersion of nanometric silver particles, including afirst solvent; a plurality of nanometric silver particles, in which amajority are single-crystal silver particles, the plurality ofnanometric silver particles having an average secondary particle size(d₅₀) within a range of 30 to 300 nanometers, the particles disposedwithin the solvent; and at least one dispersant, wherein a concentrationof the silver particles within the dispersion is within a range of 30%to 75%, by weight, and wherein a concentration of the dispersant iswithin a range of 0.2% to 30% of the concentration of the silverparticles, by weight.

According to further features in the described preferred embodiments,the dispersant concentration is at most 20%, 15%, 10%, 7%, 5%, 3%, or2%.

According to still further features in the described preferredembodiments, the viscosity of the dispersion, at 25° C., is less than2000 cP, 1000 cP, 600 cP, 300 cP, or 120 cP, and in many cases, lessthan 80 cP, 60 cP, 45 cP, 35 cP, 25 cP, or 20 cP.

According to still further features in the described preferredembodiments, the average secondary particle size is at least 40nanometers, at least 50 nanometers, at least 60 nanometers, or at least75 nanometers.

According to still further features in the described preferredembodiments, at least 60%, at least 70%, at least 75%, at least 80%, atleast 85%, or at least 90% of the nanometric silver particles aresingle-crystal silver particles.

According to still further features in the described preferredembodiments, the average secondary particle size is at most 250nanometers, at most 200 nanometers, at most 150 nanometers, at most 120nanometers, at most 100 nanometers, or at most 80 nanometers.

According to still further features in the described preferredembodiments, the at least one dispersant is selected from the group ofdispersants consisting of a polyvinylpyrrolidone (PVP), gum arabic,polyvinyl alcohol (PVA), polyacrylic acid (PAA), polyallylamine (PAAm),polysodium styrene sulfonate (PSS), 3-(aminopropyl)trimethoxysilane(APS), a fatty acid, lauryl amine, cetyltrimethylammonium bromide(CTAB), and tetraoctylammonium bromide (TOAB).

According to still further features in the described preferredembodiments, the dispersant includes PVP. The average molecular weightof the PVP is at least 8,000 gram/mole, at least 10,000 gram/mole,within a range of 10,000 gram/mole to 1,600,000 gram/mole, or within arange of 10,000 gram/mole to 200,000 gram/mole.

According to still further features in the described preferredembodiments, the first solvent includes, substantially consists of, orconsists of water.

According to still further features in the described preferredembodiments, the first solvent includes an alcohol.

According to still further features in the described preferredembodiments, the concentration of water within the dispersion is lessthan 25%, less than 15%, less than 10%, less than 7%, less than 5%, lessthan 3%, or less than 2%, by weight.

According to still further features in the described preferredembodiments, the first solvent includes at least one volatile organicsolvent.

According to still further features in the described preferredembodiments, the first solvent includes at least one non-volatileorganic solvent.

According to still further features in the described preferredembodiments, the first solvent includes at least one volatile organicsolvent and at least one non-volatile organic solvent.

According to still further features in the described preferredembodiments, the first solvent includes water and at least one volatileorganic solvent, and wherein the volatile organic solvent makes up atleast 80%, at least 85%, or at least 90% of the first solvent, byweight.

According to still further features in the described preferredembodiments, the specific electrical resistivity of the silverparticles, after standard sintering, is at most 4×10⁻⁵ ohm·cm, 6×10⁻⁶ohm·cm, at most 5×10⁻⁶ ohm·cm, at most 4×10⁻⁶ ohm·cm, at most 3.5×10⁻⁶ohm·cm, at most 3×10⁻⁶ ohm·cm, or at most 2.5×10⁻⁶ ohm·cm.

According to still further features in the described preferredembodiments, the dispersion contains at least 35%, at least 40%, atleast 45%, at least 50%, or at least 55%, by weight, nanometric silverparticles.

According to still further features in the described preferredembodiments, the secondary particle size of at least 90% of theparticles, by volume (d₉₀), is at most 500 nanometers, at most 300nanometers, at most 200 nanometers, at most 150 nanometers, at most 120nanometers, at most 100 nanometers, at most 80 nanometers, or at most 70nanometers.

According to still further features in the described preferredembodiments, the average molecular weight of the dispersant is at least8,000 gram/mole, within a range of 10,000 gram/mole to 1,600,000gram/mole, or within a range of 10,000 gram/mole to 200,000 gram/mole.

According to still further features in the described preferredembodiments, the first solvent includes at least one solvent selectedfrom the group of solvents consisting of an alcohol, dimethyl sulfoxide(DMSO), an alkylamine; ethylene diamine, dimethyl acetamide,1,4-butanediol, formic acid, acetic acid, a glycol or glycol derivative,N-methyl-2-pyrrolidone (NMP), butyl carbitol acetate, and an epoxyresin.

According to still further features in the described preferredembodiments, the glycol includes at least one of ethylene glycol,diethyleneglycol, triethyleneglycol, propylene glycol,dipropyleneglycol, and tripropyleneglycol.

According to still further features in the described preferredembodiments, the glycol derivative includes at least one of dipropyleneglycol monomethyl ether (DPM), tripropylene glycol methyl ether (TPM),and diethylene glycol monomethyl ether (DGME).

According to still further features in the described preferredembodiments, the alcohol includes at least one of ethanol, isopropanol,benzyl alcohol, and terpineol.

According to still further features in the described preferredembodiments, the alkylamine includes butylamine.

According to still further features in the described preferredembodiments, the dispersion contains less than 70%, by weight,nanometric silver particles.

According to still further features in the described preferredembodiments, the inventive dispersion is produced according to a processincluding the steps of: (a) reacting at least one soluble silvercompound with an alkali metal hydroxide in an aqueous medium, in apresence of a first dispersant, to produce silver oxide solids having anaverage secondary particle size below 1200 nanometers; (b) reacting thesilver oxide solids with at least one reducing agent in an aqueousmedium, in a presence of a second dispersant, to produce silverparticles, the silver particles having an average secondary particlesize below 1000 nanometers; and (c) providing the silver particles inthe concentrated dispersion, the concentration of the nanometric silverparticles being within a range of 30% to 75%, by weight.

According to still further features in the described preferredembodiments, the reducing agent includes, or consists substantially of,a reducing agent selected from the group consisting of peroxides andsodium borohydride.

According to still further features in the described preferredembodiments, the reducing agent includes, or consists substantially of,hydrogen peroxide.

According to still further features in the described preferredembodiments, at least one of the first and second dispersants includesPVP.

According to still further features in the described preferredembodiments, the second dispersant is added in sufficient quantitywhereby the silver particles have an average secondary particle size ofat most 250 nanometers, at most 200 nanometers, at most 150 nanometers,at most 100 nanometers, or at most 80 nanometers.

According to still further features in the described preferredembodiments, the first dispersant is added in sufficient quantitywhereby the silver oxide solids have an average secondary particle sizeof at most 200 nanometers.

According to still further features in the described preferredembodiments, the alkali metal hydroxide and the soluble silver compoundreact in a stoichiometric ratio of hydroxide to soluble silver compound,and wherein quantities of the alkali metal hydroxide and the solublesilver compound are added in a particular ratio that is, at most, 1.2times, 0.98 times, or 0.95 times the stoichiometric ratio.

According to still further features in the described preferredembodiments, subsequent to step (b), the silver particles are washed andconcentrated, whereby the aqueous medium is only partially removed fromthe particles (up to 90%, up to 80%, up to 75%, or up to 70%) to form aconcentrate.

According to another aspect of the present invention there is provided amethod for producing a dispersion of nanometric silver particles, themethod including: (a) reacting at least one soluble silver compound withan alkali metal hydroxide in an aqueous medium, in a presence of a firstdispersant, to produce silver oxide solids having an average secondaryparticle size below 1200 nanometers; (b) reacting the silver oxidesolids with at least one reducing agent in an aqueous medium, in apresence of a second dispersant, to produce a first dispersion of silverparticles, the silver particles having an average secondary particlesize below 300 nanometers; and (c) removing at least a portion of theaqueous medium from the particles to produce the dispersion.

According to further features in the described preferred embodiments,the method further includes concentrating the particles to form a seconddispersion, concentrated with respect to the first dispersion.

According to still further features in the described preferredembodiments, the second dispersion has a concentration of at least 10%,and less than 75%, by weight.

According to still further features in the described preferredembodiments, step (c) includes washing and concentrating the silverparticles, whereby the aqueous medium is partially removed from theparticles, to form a concentrate containing most of the silverparticles.

According to still further features in the described preferredembodiments, the method further includes replacing most of the aqueousmedium by at least one volatile organic solvent.

According to still further features in the described preferredembodiments, the method further includes replacing most of the volatileorganic solvent by at least one additional organic solvent.

According to still further features in the described preferredembodiments, the concentration of the nanometric silver particles withinthe dispersion is within a range of 30% to 75%, by weight.

According to still further features in the described preferredembodiments, the dispersion having any of the above-describedcharacteristics has been aged for at least 6 months, at least 9 months,at least 12 months, at least 18 months, or at least 24 months.

According to still further features in the described preferredembodiments, the reacting of the silver oxide solids is performed in thepresence of the second dispersant, an excess of which is removed in step(c).

BRIEF DESCRIPTION OF THE FIGURES

The foregoing discussion will be understood more readily from thefollowing detailed description of the invention, when taken inconjunction with the accompanying FIGS. 1-6), in which:

FIG. 1 is a schematic block diagram of a process for producing ananometric silver product, according to one aspect of the presentinvention;

FIG. 2 is a High-Resolution Scanning Electron Microscopy (HRSEM) imageshowing a typical field containing nanometric silver particles producedaccording to an embodiment of the present invention, described inExample 7;

FIG. 3 is an Electron Back Scattered Diffraction (EBSD) pattern of thenanometric silver particles produced according the embodiment describedin Example 7;

FIG. 4 is a High-Resolution Scanning Electron Microscopy (HRSEM) imageshowing a typical field containing nanometric silver particles producedaccording to an embodiment of the present invention described in Example10;

FIG. 5 is a SEM image of a sample containing nanometric silverparticles, produced according to the present invention, showing sixlocations selected for EBSD scanning; and

FIG. 6 provides five illustrations of the 3D crystal orientation for thefive locations in FIG. 5 in which monocrystalline particulate matter waspositively identified.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The principles of the inventive stable dispersions of nanometric silverparticles, and the inventive methods of producing such dispersions, maybe better understood with reference to the drawings and the accompanyingdescription.

Before explaining at least one embodiment of the invention in detail, itis to be understood that the invention is not limited in its applicationto the details of construction and the arrangement of the components setforth in the following description or illustrated in the drawings. Theinvention is capable of other embodiments or of being practiced orcarried out in various ways. Also, it is to be understood that thephraseology and terminology employed herein is for the purpose ofdescription and should not be regarded as limiting.

We have discovered a method of producing nanometric silver particles inwhich a large fraction of the particles is monocrystalline. However, wehave found the production of stable dispersions of these nanometricsilver particles to be an extremely complex, and sometimescounterintuitive, matter. The production of stable dispersions may beparticularly problematic when producing such dispersions directly invarious organic solvents that may be used in the final productdispersions. In addition, the production of stable dispersions frompre-made (e.g., commercially available) nanometric silver particles hasalso been found to be extremely difficult and unpredictable.

These challenges notwithstanding, we have surprisingly discovered amethod of purifying such nanometric silver particles while maintaining astable dispersion, whereby monocrystalline nanometric silver particlesretain their monocrystalline nature, even during contact or displacementwith those problematic organic solvents, and during the formulation ofconcentrated dispersions having at least 30% silver, by weight, and moretypically, at least 35%, at least 40%, at least 45%, at least 50%, or atleast 55% silver by weight.

Moreover, the inventive method may advantageously produce concentrateddispersions containing, mostly or predominantly monocrystalline silverparticles having an average particle size of at least 30 nanometers, andmore typically, at least 40 nanometers, at least 50 nanometers, at least60 nanometers, or at least 75 nanometers. We believe that themonocrystallinity of the silver particles is a decisive factor inattaining, after sintering, low specific electric resistivity values. Wefurther believe that the monocrystallinity of the silver particlesenables the formulation of highly concentrated dispersions of silvernanoparticles, without undergoing appreciable agglomeration.

Typically, the concentrated dispersions produced contain, at most, 10%ultrafine particles, by weight. Thus, the concentrated dispersions ofthe present invention may have a d₁₀ of at least 20 nanometers or atleast 25 nanometers, and more typically, at least 30 nanometers, atleast 35 nanometers, or at least 40 nanometers.

Although it may be possible to produce silver monocrystals according toother methods, the dispersions of the present invention may bedistinguished in at least one of several ways, including:

1. average particle size

2. fines fraction (d₁₀)

3. concentration of the silver particles within the dispersion

4. percentage of monocrystals within the silver particles, by weight.

1. Average Particle Size

In the concentrated dispersions of the present invention, the nanometricsilver particles having an average secondary particle size (d₅₀) withina range of 30 to 300 nanometers. These dispersions contain, mostly orpredominantly monocrystalline silver particles.

It must be emphasized that the production of a monocrystal having adiameter of 60 nanometers is over 200 times the size of a monocrystalhaving a diameter of 10 nanometers [60/10){circumflex over ( )}3=2161].The growth of a monocrystal from a diameter of 10 nanometers to 60nanometers within a solvent requires over 200 times the deposition ofsupersaturated silver onto a 10 nanometer monocrystal, than was requireto initially form the 10 nanometer monocrystal.

In crystallization processes, competing mechanisms, including nucleationand agglomeration, may interfere, or even predominate, with respect tocrystal growth. In the case of reactive precipitation processes, inwhich sparingly soluble materials and the like are precipitated out ofsolution in a reactive process, these competing processes may be greatlyfavored, particularly in view of the extremely high globalsupersaturation levels, and even higher local supersaturation levels.These competing processes may further be favored due to the presence ofsolid reactant particles, around which the local supersaturation may beeven higher, favoring nucleation and agglomeration over crystal growth.In the methods of the present invention, the silver particles areprecipitated out of solution in such a reactive process. Hence, theproduction of silver particles that are mostly or predominantlymonocrystalline silver particles would appear to be, at the very least,unexpected. The production of a monocrystalline silver product having ad₅₀ of 30 nanometers, and typically more, is surprising.

2. Fines Fraction (d₁₀)

The silver particles in the dispersions of the present invention may befurther characterized by the paucity of fine silver particles. Using aBrookhaven 90 Plus particle size analyzer, the dispersions of thepresent invention exhibited a d₁₀ of at least 25 nanometers, and moretypically, at least 30 nanometers, at least 35 nanometers, and in somecases, at least 40 nanometers. The data from the Brookhaven particlesize analyzer was subsequently confirmed by measurements based on HighResolution Scanning Electron Microscopy (HRSEM) images.

Thus, at least 90%, by weight, of the silver particles in thedispersions of the present invention have a diameter of at least 25-40nanometers.

3. Concentration of the Silver Particles Within the Dispersion

The concentration of nanometric silver particles within the concentrateddispersions of the present invention is typically within a range of 30%to 75%, by weight. The production of the nanoparticles is typicallyperformed whereby a relatively dilute dispersion is obtained. The workupof the dilute dispersion, which may include washing, solvent additionand/or replacement, etc., may be a major contributor to theagglomeration of silver nanoparticles. In the inventive process,described hereinbelow, the agglomeration of the silver nanoparticles islargely averted.

4. Percentage of Monocrystals Within the Silver particles, by Weight

In the concentrated dispersions of the present invention, the nanometricsilver particles may be mostly or predominantly monocrystalline silverparticles, on a weight basis. The presence of monocrystals wasqualitatively demonstrated by means of Electron Back ScatteredDiffraction (EBSD). Quantification of the results was achieved byperforming a plurality of scans at randomly chosen points, as describedin greater detail hereinbelow. We have found that in our inventivedispersions, at least 50% or at least 70% of the nanometric silverparticles are monocrystalline, and more typically, at least 80% or atleast 90% are monocrystalline.

Referring now to the drawings, FIG. 1 is a schematic block diagram of amethod of producing a nanometric silver product, according to one aspectof the present invention. The method may include the following steps:

Step 1: reacting at least one soluble silver compound with an alkalimetal hydroxide in an aqueous medium, in a presence of a firstdispersant, to produce silver oxide solids having an average secondaryparticle size below 1200 nanometers;

Step 2: reacting the silver oxide solids with at least one reducingagent in an aqueous medium, in a presence of a second dispersant, toproduce silver particles, the silver particles having an averagesecondary particle size below 500 nanometers, and more typically, below300 nanometers;Step 3: purifying the silver particles of Step 2 by washing with water;the aqueous medium may also be partially removed from the particles, toform a concentrate containing most of the silver particles;Step 4: introducing, to the purified silver particles, at least onevolatile organic solvent, and replacing most of the aqueous mediumtherewith; andStep 5: replacing most of the volatile organic solvent by at least one,typically non-volatile, organic solvent.

Various embodiments of the inventive method of producing a nanometricsilver product will now be described in further detail.

Step 1

At least one soluble silver compound is dissolved in an aqueous solventto form a first solution. The alkaline hydroxide (e.g., sodium orpotassium hydroxide) may then be added, under vigorous stirring to thisfirst solution. However, it may be advantageous to prepare a secondsolution of the alkaline hydroxide. The second solution may then beintroduced to the first solution, under vigorous stirring, and in thepresence of a dispersant, to form a fine silver oxide precipitate. Theresulting dispersion is preferably agitated, and an anti-foaming agentmay be added to prevent or reduce foaming.

The vigorous mixing may be effected in an ultrasonic bath, typicallymaintained within a temperature range of 10° C. to 35° C.

Various and varied dispersants may be used to contribute to the qualityof the inventive nanometric silver product, includingpolyvinylpyrrolidone (PVP), gum arabic, polyvinyl alcohol (PVA),polyacrylic acid (PAA), polyallylamine (PAAm), polysodium styrenesulfonate (PSS), 3-(aminopropyl)trimethoxysilane (APS), fatty acids,such as stearic, palmitic, lauric etc.; lauryl amine,cetyltrimethylammonium bromide (CTAB), tetraoctylammonium bromide(TOAB).

The use of PVP has been found to be particularly advantageous. While PVPhaving an average molecular weight of up to about 2,000,000 gram/molemay be used, we have found to be particularly effective, in most cases,PVP molecules having an average molecular weight higher than about 8,000gram/mole, and more typically, an average molecular weight within therange of 10,000 gram/mole to 200,000 gram/mole. The weight ratio of thePVP to the silver particles, in Step 1, is typically within the range of0.01 to 10, and more typically, within the range of 0.1 to 5.

An anti-foaming agent may be introduced to control foaming in any of theprocess steps, as necessary.

Step 2

A reducing agent may be added, under vigorous mixing, to a dispersioncontaining fine silver oxide particles, whereby the oxide particles arereduced, producing a second dispersion containing nanometric silverparticles. The presence of a dispersant in Step 2 may reduce orsubstantially inhibit agglomeration. The dispersant may be thepreviously added dispersant from Step 1, or may be a mixture of thepreviously added dispersant from Step 1 and a newly added dispersant.The newly added dispersant may be identical to the dispersant used inStep 1, or may be a different chemical species.

The vigorous mixing may be effected in an ultrasonic bath, typicallymaintained within a temperature range of 10° C. to 35° C.

We have found that in producing the dispersion of nanometric silverparticles of the present invention, it may be essential to reducefreshly precipitated silver oxide solids, which have been precipitatedin the presence of a suitable dispersant, as described in Step 1. In ourattempts to circumvent Step 1 by using commercially available silveroxide solids as raw material for Step 2, the dispersions of nanometricsilver particles had different and generally disadvantageous physicalproperties with respect to those obtained by the methods of the presentinvention. We have found this to be the case even when a suitabledispersant is pre-introduced to the reaction mixture (containing thecommercially available silver oxide solids) of Step 2.

Preferably, the reduction reaction is conducted within an ambienttemperature range (typically between 10° C. and 35° C.) to obtain thedesired dispersion of nanometric silver particles. However, thereduction reaction temperature may be allowed to rise to about 60° C.without adversely affecting the resulting product properties. We notethat within this temperature range, an alcohol (such as ethanol orethylene glycol) is incapable of converting the silver ions intosilver(0), such that the reduction is accomplished solely by thereducing agent or agents.

These reducing agents may include a peroxide, ascorbic acid, sugars suchas glucose, metal hydrides such as sodium borohydride, hydrazinehydrate, formaldehyde, and a saccharide, or reducing agents belonging tothe chemical families of these reducing agents. Hydrogen peroxide, whichmay be considered a “green” reagent, in that the decomposition thereofultimately leads to the formation of water and oxygen, may be apreferred peroxide.

In order to simplify the method of the invention, the dispersant used inStep 1 may be reused as the dispersant for Step 2. Additionaldispersants may be introduced in Step 2, including those dispersantsmentioned hereinabove with respect to Step 1.

Yield and economic considerations would appear to dictate astoichiometric ratio of at least 1 to 1 between the alkali hydroxide andthe soluble silver salt (such as silver nitrate) reacted in Step 1. Inthis manner, the yield of the silver oxide intermediate product would behighest. Surprisingly, however, we have discovered that using astoichiometric excess of alkali hydroxide may ultimately result in heavyagglomeration of the silver particles. In the laboratory, a 50% excessof an alkaline hydroxide (such as potassium hydroxide) with respect tothe stoichiometric ratio of hydroxide to soluble silver compounds,resulted in disadvantageous agglomeration of the silver particles.Similar results were obtained for close to a 20% excess of the alkalinehydroxide with respect to the stoichiometric ratio of the alkalinehydroxide to silver nitrate.

We have found advantageous, after a course of lengthy experimentation,to operate Step 1 at a stoichiometric ratio of hydroxide to a solublesilver compound within a narrow range between 0.8 to 1.0, and moretypically, between 0.8 to 0.98 or between 0.8 to 0.96. Within thisrange, the yield of silver is indeed lower, but the yield ofhigh-quality product may be greatly increased.

In Step 2, the concentration of silver particles within the reactionmixture is typically between 0.5% to 5%, by weight, and more typically,between 1% to 3%, by weight.

In addition to water, an additional solvent can be introduced in Step 1and/or Step 2. Typically, the additional solvent includes a polarsolvent such as a polar organic solvent. It is generally advantageousfor the additional solvent to be relatively volatile, soluble in water,and to dissolve substantial amounts of the dispersants used.

Preferably, alcohols like methanol, ethanol, and isopropyl alcohol (IPA)can be used as polar solvents. However, various glycols and the like mayalso be used.

Step 3

Typically, water or an aqueous solvent may be used to purify theresulting dispersion of Step 2 in a suitable purification system. Theintroduction of water or aqueous solvent to the purification system iscontrolled to replace the spent aqueous liquor, while maintaining theconcentration of the silver particles, at any time, below a preset value(below 90%, by weight, and preferably below 80%, below 70%, or below60%). As a result, substantially all the salts and most of thedispersant in the aqueous liquor are removed without deleteriouslychanging the shapes or agglomerating the silver particles.

The aqueous solvent may contain, in addition to water, an organicsolvent such as a polar organic solvent. The streams produced in Step 3typically include a concentrate containing most of the nanometric silverparticles, and a relatively dilute stream containing a lowerconcentration of the silver nano-particles, and preferably, containingsubstantially none of the silver nano-particles. In Step 3,substantially all of the salts, part of the dispersant, and part of theliquid that are present with the formed silver particles are removed.Usually, particular values of the final concentration of the salts(based on the silver weight), of the dispersant (based on the silverweight), and of the silver particles (based on the dispersion weight)are preset, and the operation of Step 3 is considered finished whenthese preset values are met.

We have found that Step 3 can be conducted in a microfiltration systemsuch as a membrane purification system having at least one membranecapable of separating the silver particles from the aqueous liquor,without losing a fraction of the silver particles in the aqueous phasethat would make the process economically unviable. Alternatively oradditionally, Step 3 may be conducted in a centrifuge purificationsystem having at least one centrifuge, such as a decanter centrifuge.

A microfiltration system and method of general relevance to the presentinvention is disclosed by Pagana et al., “Applied Pilot-Scale Studies onCeramic Membrane Processes for the Treatment of Wastewater Streams”(Global NEST Journal, Vol. 8, No. 1, pp 23-30, 2006), and isincorporated in its entirety by reference into the specification, as iffully set forth herein.

At least one membrane of the membrane purification system should becapable of filtering off the nanometric silver particles in thedispersion. To this end, the characteristic pore size of this membranemay be within a range that is suitable to retain the nanometric silverparticles. The membranes may be made of a metallic material, ceramicmaterial, polymeric material, or of other materials that may be known tothose of ordinary skill in the art.

Step 4

A volatile organic solvent may replace most of the aqueous liquor of thepurified dispersion obtained in Step 3, in a method similar to themethod utilized in Step 3. The same purification system may be used. Indisplacing the aqueous liquor, a further purification of the silverparticles is achieved, which may be essential for various products andapplications.

The volatile organic solvent may advantageously be soluble in water, andmay readily dissolve the dispersant or dispersants remaining from Step3. Various solvents may be appropriate as solvents for Step 4 of theprocess, either alone or mixed with at least one additional solvent.These solvents include, but are not limited to alcohols such asmethanol, ethanol, propanol, isopropanol, and a butanol such as1-butanol; acetonitrile; dimethyl sulfoxide (DMSO); alkylamines such asbutylamine; ethylene diamine; dimethyl acetamide; 1,4-butanediol; formicacid; and acetic acid.

Step 5

A second organic solvent, whose identity and properties may be dictatedby market requirements, may be used to replace most of, and typically atleast 80% of, or at least 90% or 95% of, the volatile organic solvent ofthe dispersion obtained in Step 4. The method of solvent replacement ordisplacement may be similar to the method utilized in Step 3 and/or Step4, and the purification system may be similar or identical.

However, the second organic solvent may replace the volatile organicsolvent in an evaporation system in which the volatile organic solventis evaporated, with a concomitant addition of the desired organicsolvent, in order to maintain a concentration of the silver particlesbelow a particular, desired value. Typically the concentration of thesilver particles is at most 90%, at most 85%, or at most 80%.

Various solvents may be appropriate as solvents for Step 5 of theprocess, either alone or mixed with at least one additional solvent.These solvents include, but are not limited to, ethylene glycol andderivates thereof (e.g., diethylene glycol monomethyl ether (DGME),dipropylene glycol (DPG), dipropylene glycol monomethyl ether (DPM), andtripropylene glycol methyl ether (TPM)); N-methyl-2-pyrrolidone (NMP);various alcohols, including ethanol, isopropanol, benzyl alcohol, andterpineol; butyl carbitol acetate; and specific epoxy resins. Oneappropriate solvent mixture is TPM/NMP, which may typically be used upto a weight ratio of about 85:15.

Various solvents were found to be less suitable or unsuitable for use inStep 5, including acetates such as propylene glycol methyl ether acetate(PMA), which caused agglomeration of the silver particles when used inconjunction with certain dispersants or surfactants.

Thus, by preparing the nanometric silver particles as described in Steps1 and 2, and by conducting the work-up procedure as described in Steps3-5, the nanometric silver dispersions of the present invention mayachieve exceptional stability (having a guaranteed shelf-life of atleast 9 months, and more typically, at least 12 months, at least 18months or at least 24 months). The inventive dispersions may becharacterized by very low specific resistivity values (at most 6×10⁻⁶ohm·cm, at most 5×10⁻⁶ ohm·cm, at most 4×10⁻⁶ ohm·cm, at most 3.5×10⁻⁶ohm·cm, at most 3×10⁻⁶ ohm·cm, or at most 2.5×10⁻⁶ ohm·cm), as measuredaccording to ASTM procedure F 390-98 (reapproved 2003).

In another embodiment of the present invention, we have discovered thatthe first purifying step (Step 3) may advantageously be effected bynano-separation or nano-filtration using nano-separation membranes. Suchprocesses may be excessively and impractically slow when the size of thefiltrate species approaches that of the membrane pore or opening.Moreover, nano-filtration membrane processes may even be substantiallyimpossible when the size of the filtrate species equals, or exceeds, thesize of the membrane opening. A further, perhaps even more significantdeterment to using such nano-separation processes relates to therelative size between the particles or species impeded by thenano-separators (such as nano-membranes), and the species that aresupposed to pass through the nano-separators. We have discovered thatcertain dispersants, such as PVP, may have an elongated or needle-likestructure. While the characteristic long dimension or diameter of suchmolecules is much too large to pass through the openings ofnano-separators, and may be considerably larger than the silvernano-particles themselves, the characteristic narrow dimension ordiameter of such molecules may be orders of magnitude smaller. Thus, thestructure of the dispersant can be tailored to satisfy the processrequirements for advantageously effecting a nano-separation of thedispersant from the silver nano-particles.

Thus, according to one preferred embodiment of the present invention,the PVP dispersant has at least one narrow characteristicdimension/diameter with respect to the silver particles and with respectto the characteristic diameter of the membrane openings. This narrowcharacteristic dimension/diameter of the dispersant is preferably lessthan half the average secondary particle size of the silver particles.

Thus, while the average molecular weight of the polyvinylpyrrolidoneshould typically have an average molecular weight of less than about200,000 grams per mole, in order to pass through various suitablenano-membranes, the average molecular weight of the polyvinylpyrrolidoneshould preferably exceed about 8,000 grams per mole, to avoid reactivityand/or compatibility issues in either or both of the reaction steps.Despite the reduced separation efficiency, it is generally preferablefor the PVP to have an average molecular weight of at least 15,000, atleast 20,000, or even at least 25,000 grams per mole.

In some applications, e.g., in which extremely fine silver particles areproduced, or in which elevated separation kinetics are desirable, thePVP should preferably have an average molecular weight of less thanabout 100,000 grams per mole, and more preferably, less than about80,000 grams per mole.

Ceramic nano-filtration membranes have been advantageously employed, butpolymeric and/or metallic nano-filtration membranes may also befundamentally suitable. The membrane systems may be static or dynamic(e.g., having a vibrational mechanism for facilitating the separation).

Typical ceramic nano-separation or nano-filtration membranes for use inconjunction with the method of the present invention have one or morepores that are typically cylindrical, with a high length to width aspectratio, through which the water/solvent and fine matter can pass through.In many cases, the membrane is typically shaped like a long cylinder,but other geometries may be practical.

We have found that nano-membranes having a pore diameter of less than200 nanometers may be suitable for use in the process of the presentinvention. In some applications, the preferred pore diameter is lessthan 150 nanometers, less than 120 nanometers or less than 100nanometers. Generally, the pore diameter or nominal pore diameter of themembrane may be at least 20 nanometers, and often, at least 30-50nanometers, so as to enable various species to pass through the membraneopenings, and so that the separation kinetics are not prohibitivelyslow.

According to another preferred embodiment of the present invention, thedispersant size and shape and the size of the membrane openings may beselected such that the silver nano-particles pass through the openings,while passage of the dispersant through the openings is hindered orsubstantially prevented.

Downstream of the separation step, much smaller membrane openings may beselected, such that the passage of the silver nano-particles through theopenings is hindered or substantially prevented, while smaller moleculessuch as water, ethanol, etc., pass through the openings with relativefacility. This embodiment of the present invention may be particularlyefficacious in applications in which the average particle size of thesilver product is particularly low, or in which a substantial fractionof the silver product has a low average secondary particle size (e.g.,below 30 nanometers, or even below 50 nanometers).

In another preferred embodiment of the present invention, the formationof silver oxide and reduction of the silver oxide to produce the silvernano-crystals is performed in a single process step. However, thespecific conditions and preferred reagents and dispersants aresubstantially similar to those provided hereinabove for the two-stepreaction process. By way of example, an aqueous solution containingpotassium hydroxide may be introduced under vigorous stirring to asecond aqueous solution containing a soluble silver compound such assilver nitrate, a dispersant such as PVP, and a reducing agent such as ahydrogen peroxide. In this case, the reduction of the silver ions onlycommences when the hydroxide solution mixes with the second aqueoussolution.

EXAMPLES

Reference is now made to the following Examples, which together with theabove description, illustrate the invention in a non-limiting fashion.

The chemicals used in effecting these Examples are identifiedhereinbelow:

AgNO₃-Aldrich

AgNO₃ (containing 63.6% Ag)-Saxonia (Germany)

KOH-Aldrich

hydrogen peroxide (aqueous solution of ˜33%)-Makhteshim (Israel)

polyvinylpyrrolidone (PVP), MW=55,000-Aldrich.

polyvinylpyrrolidone (PVP), MW=8,000-Aldrich.

anti-foaming agent Contraspum 1012-Zschimmer & Schwarz (Germany)

ethanol abs.-Aldrich

isopropyl alcohol (IPA)-Aldrich

tripropyleneglycol methyl ether (TPM)-Aldrich

butyl carbitol acetate (BCA)-Aldrich

caprylic acid-Aldrich

Epoxy XY8000-Japan Epoxy Resins Co., Ltd. (Japan). Epoxy XY8000 may beidentified by CAS No. 30583-72-3, and has the chemical namecyclohexanol, 4,4-(1-methylethylidene)bis-, polymer with(chloromethyl)oxirane.

Aqueous solutions were prepared by using deionized water using an Ionexwater purification system (PuriTech, Dessel, Belgium). All reagents andsolvents were used without further purification.

The instruments used in conjunction with the Examples are identifiedhereinbelow:

Particle size analyses (d₅₀) were performed using a Brookhaven 90 Plusparticle size analyzer (Brookhaven Instruments Corporation, Holtsville,N.Y.).

The particle size analysis (d₅₀) in Example 3 was performed using aMalvern Master Sizer 2000.

High Resolution Scanning Electron Microscopy (HRSEM) images weregenerally obtained using an HRSEM Ultra Plus Zeiss Gemini (InlensDetector).

Electron Back Scattered Diffraction (EBSD) Patterns were obtained usingan E-SEM Quanta™ 200 (FEI, Hillsboro, Oreg.). The instrument wasequipped with an attachment for orientation image microscopy (OIM)Channel 5 (Oxford Instruments, England).

Evaporation was performed using a R-215 Rotavapor® equipped with aheating bath (BÜCHI Labortechnik AG, Flawil, Switzerland).

Filtration of dispersions was conducted by using a membrane system thatincluded ceramic membranes (JM Separations BV, The Netherlands).

Example 1

52 g AgNO₃ and 3.3 g PVP (MW=55,000) were dissolved in a mixture of 780ml ethanol and 80 ml water (solution A). 17 g KOH were dissolved in 140ml water (solution B). Solution B was poured into solution A undervigorous stirring in an ultrasonic bath, forming a colloidal precipitateof Ag₂O at ambient temperature. After stirring the dispersion for 10minutes, 180 ml H₂O₂ (33%) were slowly pumped into the dispersion understirring at the temperature range between about 25° C. to about 60° C.,forming silver nanoparticles. The dispersion was stirred for additional15 minutes, and was transferred to a storage tank to await furthertreatments.

A particle size analysis yielded an average particle size (d₅₀) of about80 nanometers.

Example 2

53 g AgNO₃ and 54 g PVP (MW=55,000) were dissolved in 860 ml water(solution A). Five drops of an anti-foaming agent were also introduced.17 g KOH were dissolved in 140 ml water (solution B). Solution B waspoured into solution A under vigorous stirring at ambient temperature,forming a colloidal, nanometric precipitate of Ag₂O (d₅₀ under 60nanometers). After stirring the dispersion for 10 minutes, 180 ml H₂O₂(33%) were slowly pumped directly into the dispersion under stirring, insitu, reducing the silver oxide to silver, at which time the reactionmixture heated up from about 25° C. to about 60° C. The dispersion wasstirred for an additional 15 minutes, and was transferred to a storagetank to await further treatments.

A particle size analysis of the silver particles yielded an averageparticle size (d₅₀) of about 50 nanometers.

Example 3

Two liters of an aqueous solution containing 170 g/l AgNO₃ and 90 g/lhydrogen peroxide (33%) were dripped, under intensive stirring, into oneliter of aqueous solution of KOH having a concentration of 56 g/l, atwhich time the reaction mixture heated up from about 25° C. to about 60°C. The silver particles produced were agglomerates.

A particle size analysis yielded an average particle size (d₅₀) of about1.5 microns (1500 nanometers).

Example 4

One liter aqueous solution of KOH having a concentration of 56 g/l wasdripped, under intensive stirring, into two liters of an aqueoussolution containing 170 g/l AgNO₃, 90 g/l hydrogen peroxide (33%), and170 grams PVP (MW=55,000). During the reaction, the temperatureincreased in the range between about 25° C. to about 60° C. Thedispersion was stirred for an additional 15 minutes, and was transferredto a storage tank to await further treatments.

A particle size analysis of the silver particles yielded an averageparticle size (d₅₀) of about 90 nanometers.

Example 5

53 g AgNO₃ and 100 g PVP (MW=8,000) were dissolved in 850 ml water(solution A). Five drops of an anti-foaming agent were also introduced.17 g KOH were dissolved in 140 ml water (solution B). Solution B waspoured into solution A under vigorous stirring, forming a colloidalnanometric precipitate of Ag₂O. After stirring the dispersion for 10minutes, 180 ml H₂O₂ (33%) were slowly pumped directly into thedispersion under stirring, in situ, reducing the silver oxide to silverat the temperature range between about 25° C. to about 60° C.

The silver particles produced were agglomerates.

Example 6

Example 2 was repeated, but using three-fold quantities. 159 g AgNO₃ and162 g PVP (MW=55,000) were dissolved in 2580 ml water (solution A).Fifteen drops of an anti-foaming agent were also introduced. 51 g KOHwere dissolved in 420 ml water (solution B). Solution B was poured intosolution A under vigorous stirring, forming a colloidal nanometricprecipitate of Ag₂O. After stirring the dispersion for 10 minutes, 540ml H₂O₂ (33%) were slowly pumped directly into the dispersion understirring, in situ, reducing the silver oxide to silver at a temperaturerange between 25° C. and about 60° C. The dispersion was stirred for anadditional 15 minutes, and was transferred to a storage tank to awaitfurther treatments.

A particle size analysis of the silver particles produced yielded anaverage particle size (d₅₀) of about 50 nanometers of a relativelynarrow distribution.

Example 7: Concentrating a Dispersion

1000 ml of the product dispersion from Example 6, containing about 25 gof the nanometric silver particles, were pumped out from the top of thestorage tank at which it was kept without any mixing, to assure thatlarge particles, if any are therein, are precipitated onto the bottom ofthe storage tank, and are avoided in the subsequent treatments. Thedispersion was washed in a membrane separation system by feedinggradually and continuously about 20 liters water into the membranesystem, and simultaneously, by gradually and continuously withdrawing asimilar volume of spent wash liquor from the membrane system in such amanner that the concentration of the silver particles, never exceeded90% (and preferably less than 60%), on a weight basis. The membranesystem included ceramic membranes (JM Separations BV) having separationpores or capillaries having a nominal pore diameter of 100 nanometers.Water was added, and ionic matter and dispersant, selectively, passedthrough the membranes, leaving the nanometric silver particles in thedispersion.

The washing process was continued until the salts in the dispersionwere, practically eliminated, and the dispersant was reduced to a presetconcentration of 3% of the weight of the silver particles.

As the mass balance of the water feed to the membrane system and thespent wash liquor flowing out of the membrane system was, deliberately,changed along this washing step in order to effect an efficient washingoperation, and in order to obtain a concentrated dispersion, theresulting washed silver dispersion contained about 25% solids, on aweight basis. A particle size analysis of the washed silver particlesyielded an average particle size (d₅₀) of about 50 nanometers; nosignificant change in particle size between the unwashed and washednano-silver product was observed.

The HRSEM image of the nanometric particles obtained is provided in FIG.2 (instrument magnification=×100,000; image viewingmagnification=×40000), and the Electron Back Scattered Diffraction(EBSD) pattern of these silver particles is given in FIG. 3. Thesefigures reveal the following characteristics with regard to the silverparticles:

1. A majority of single-crystal silver particles (typically, at least70%, at least 80%, or at least 90%, as determined by EBSD correlation);

2. The single-crystal particles include particles having triangularfaces, square faces, hexagonal faces, and heptagonal faces; and

3. The single-crystal particles having triangular faces represent atleast 2%, at least 5%, and typically, between 2% to 15% of the silverparticles, based on the number of particles (determined by manualcounting of particles in SEM fields).

Example 8: Replacing Water with a Volatile Organic Solvent(Water-Ethanol Solvent Exchange)

A 1000 ml portion of a dispersion of silver particles in water,containing about 150 g of the silver particles, which was prepared in asimilar manner to that in Example 7, was concentrated to 500 ml usingthe same membrane separation system as in Example 7. 400 ml ethanol werethen added, and the dispersion was concentrated once again to 500 ml bywithdrawing the requisite volume of liquid. This cycle, in which 400 mlethanol is added and about 400 ml ethanol-water mixture is withdrawn,was repeated until the concentration of the ethanol reached 94%-95% byweight (which is close to the composition of an azeotropic mixture ofethanol-water). The resulting 500 ml silver dispersion contained about150 g of nanometric silver and about 300 g of the ethanol-water mixture.

A particle size analysis of the silver dispersion after thewater-ethanol exchange yielded an average particle size (d₅₀) of about80 nanometers.

A majority of the particles obtained were monocrystals.

Example 9: Replacing Water with a Volatile Organic Solvent (IPA)

Example 8 was repeated with isopropyl alcohol (IPA) instead of ethanol.

A particle size analysis of the silver dispersion after thewater-isopropyl alcohol exchange yielded an average particle size (d₅₀)of about 90 nanometers.

A majority of the particles obtained were monocrystals.

Example 10: Replacing the Volatile Organic Solvent with an OrganicNon-Volatile Solvent (Ethanol-TPM Solvent Exchange)

The 500 ml of silver dispersion from Example 8, containing about 150 gof silver particles and about 300 g of solvent (ethanol-water mixture),were transferred into a 1-liter flask. 150 gtripropyleneglycolmethylether (TPM) were added to the flask (in order toultimately obtain a final dispersion containing about 50% solids, on aweight basis). The flask was connected to a Rotavapor® apparatus, andthe ethanol was evaporated under vacuum (at 20 mm Hg; 60° C.; 80 rpm).The resulting silver dispersion in TPM contained 49.5% (by weight)silver with an average particle size (d₅₀) of about 50 nanometers (aswell as traces of ethanol and water). This specific dispersion displayeda shelf-life exceeding one year (and exceeding even two years).Moreover, after drying and thermal sintering, the specific resistivity,as measured according to the ASTM standard procedure F390-98, was below4×10⁻⁶ ohm·cm, which is considered to be an excellent value forapplications such as conductive inks.

A HRSEM image showing the nanometric silver particles obtained isprovided in FIG. 4 (instrument magnification=×100,000; image viewingmagnification=×40000). It is evident that the general appearance of thesilver particles has not been noticeably changed with respect to that ofthe particles obtained in Example 7.

The viscosity of the dispersion was measured, for various concentrationsof silver particles (metal loading). The results are provided in Table1.

TABLE 1 Metal loading (%)* 45 48 49 50 53 55 59 Viscosity at 25° C. 3040 50 60 85 140 460 (cP) *Metal loading % is defined as the weight ofmetal (silver) particles × 100 divided by the weight of the dispersion.

Example 11: Replacing the Volatile Organic Solvent (IPA) with an OrganicSolvent-BCA (IPA-BCA Solvent Exchange)

The 500 ml of silver dispersion from Example 9, containing about 150 gof silver particles and about 300 g of solvent (IPA-water mixture) weretransferred into a 1-liter flask. 150 g butyl carbitol acetate (BCA)were added to the flask (in order to obtain, eventually, a finaldispersion containing about 50% solids, on a weight basis). The flaskwas connected to a Rotavapor® apparatus, and the IPA was evaporatedunder vacuum (at 20 mm Hg, 60° C., and 80 rpm). The resulting silverdispersion in BCA contained 49.7% (by weight) silver particles having anaverage particle size (d₅₀) of about 60 nanometers. The silverdispersion also contained traces of IPA and water.

Example 12: Replacing the Volatile Organic Solvent (IPA) with an EpoxyResin (IPA-Epoxy Resin Exchange)

The 500 ml of silver dispersion from Example 9, containing about 150 gof silver particles and about 300 g of solvent (IPA-water mixture), weretransferred into 1 liter flask, and 15 g caprylic acid were added. Theflask was connected to a Rotavapor® apparatus, which was rotated for 15minutes at 80° C., at 80 rpm). Subsequently, 150 g Epoxy XY8000 resinwere added to the flask (in order to obtain, eventually, a finaldispersion containing about 50% solids, on a weight basis). TheRotavapor was reactivated (at 20 mm Hg, 80° C., and 80 rpm), and after 1hour, most of the IPA-water solvent was evaporated.

The resulting silver dispersion in the epoxy resin contained 48% (byweight) silver particles having an average particle size (d₅₀) of about70 nanometers. The silver dispersion also contained traces of IPA andwater.

A majority of the particles obtained were monocrystals.

Example 13

The presence of monocrystals was qualitatively demonstrated by means ofElectron Back Scattered Diffraction (EBSD).

EBSD produces a diffraction pattern from the surface of the sample ofsilver nanoparticles. The procedure, which will be readily understood tothose of ordinary skill in the art of EBSD, is as follows:

-   -   1. The sample is scanned using a scanning electron microscope        (SEM) Quanta™ 200, typically at a working distance of 18 mm, and        at 20 KeV, to obtain a diffraction pattern or image. The spot        size is 4.5; the probe current is about 0.5 nA; the collection        time for EBSD pattern: 300 msec; integration: 50.    -   2. Interpretation of the diffraction image is performed using        the instrument software (comparing basic silver crystallographic        data);    -   3. The “solution” of the diffraction is matched, depicting the        orientation of the crystal, correlating every kikuchi line to        its fitting crystallographic plane in the lattice. If there is a        perfect match between the kikuchi lines and the crystallographic        planes (according to the theoretical data), the diffraction        determines the orientation of a single crystal.

In the case of nanometric silver particles, a perfect solution may notalways be obtained; sometimes there is no solution at all. This mayindicate that the beam is located on a grain boundary. Alternatively, aperfect solution may not be obtained when the beam is located betweentwo grains.

Example 14

Following the basic procedure of Example 13, we quantified the presenceof nanometric silver monocrystals within each sample of nanometricsilver. Quantification was achieved by performing a plurality (at least5, and preferably at least 10) of scans at randomly chosen points. Intesting the various dispersions of the present invention, at least 30%or at least 50% of the scans produce a substantially perfect match for asilver single crystal. More typically, at least 80%, at least 90%, orsubstantially 100% of the scans produce a substantially perfect matchfor a silver single crystal.

We have found that if at least 30% of the scans produce a substantiallyperfect match for a silver single crystal, then a majority of thenanometric silver particles are monocrystalline (based on the number ofparticles). If at least 50% of the scans produce a substantially perfectmatch for a silver single crystal, then at least 60%, and typically atleast 70% of the nanometric silver particles are monocrystalline. If atleast 60% of the scans produce a substantially perfect match for asilver single crystal, then at least 70%, and typically at least 80% ofthe nanometric silver particles are monocrystalline. If at least 80% ofthe scans produce a substantially perfect match for a silver singlecrystal, then at least 90%, and typically at least 95% of the nanometricsilver particles are monocrystalline.

In theory, these quantitative EBSD scanning methods may provide aquantitative evaluation of a top layer or cross-section of the sample.In practice, however, this quantitative evaluation closely reflects thefraction of silver particles having monocrystalline character,particularly for samples that do not have an extremely broad particlesize distribution.

Example 15

FIG. 5 is a SEM image of a sample containing nanometric silverparticles, produced according to the present invention. Randomly chosenlocations in the sample were scanned. In five of the six locations, aperfect match for a silver single crystal was obtained.

FIG. 6 provides five illustrations of the 3D crystal orientation for thefive locations in which a perfect match was obtained. Each one of thediffractions produced had a different orientation. Moreover, theorientation distribution obtained was very close to the theoreticalrandom distribution, indicating that no preferred orientation in thenanometric silver particles of the sample.

Example 16

Inventive dispersions of silver particles were prepared for specificresistivity testing as follows:

The dispersion is dropped on a glass substrate having dimensions of ˜3cm×3 cm, by means of a pipette, until the substrate is fully covered.Heat treatment (in air) is effected at 130° C. for 10 minutes, followedby 640° C. for 20 minutes. The thermally-sintered sample is immediatelyremoved from the oven. Typically, the film thickness is about 10micrometers.

Using a four-point probe measurement, the specific resistivity isobtained.

Example 17

An inventive dispersion of silver particles in tripropyleneglycolmonomethyl ether (TPM) was prepared for specific resistivity testingaccording to the procedure provided in Example 16. The sample had ametal loading of approximately 50%.

The specific resistivity, determined according to the procedure providedin Example 16, was about 3.5×10⁻⁶ ohm·cm, just slightly more than twicethe specific resistivity of bulk silver.

The term “average secondary particle size”, used herein, in thespecification and in the claims section that follows, is used withregard to the silver oxide and the silver particles, and refers to themean diameter of the silver oxide and silver particles, and isspecifically meant to include the diameters of agglomerated particles.

As used herein in the specification and in the claims section thatfollows, the term “mean diameter”, used with regard to silver oxide andsilver particles, refers to an equivalent spherical particle size (d₅₀),calculated using the Stokes-Einstein equation, by a Brookhaven 90 Plusparticle size analyzer (Brookhaven Instruments Corporation, Holtsville,N.Y.), or if unavailable, by a functionally-equivalent particle sizeanalyzer suited for measuring equivalent spherical particle sizethroughout the range of 5 to 2000 nanometers.

In determining this mean diameter, the particle size analysis isperformed in a professional and reproducible manner using the particlesize analyzer, by personnel trained and qualified to operate theparticle size analyzer, and under the following conditions:

-   -   (1) a representative sample of the solid particles (silver        oxide, silver) is taken;    -   (2) the analysis is performed in a dispersion of the solid        particles in its respective liquid;    -   (3) the analysis is performed at room temperature;    -   (4) the scattering angle is 90 degrees.

As used herein in the specification and in the claims section thatfollows, the term “polyvinylpyrrolidone”, also known as PVP, refers to awater-soluble polymer having or including the following molecularstructure:

PVP is typically made from the vinyl pyrrolidone monomer, which has thefollowing structure:

The market of PVP dispersants includes polymers produced by attaching(e.g., grafting) PVP onto other moieties. As used herein in thespecification and in the claims section that follows, the term“polyvinylpyrrolidone” includes such dispersants.

As used herein in the specification and in the claims section thatfollows, the term “silver compound”, and the like, is meant to includean inorganic silver salt, an organic silver salt, or an organo-silvercomplex.

As used herein in the specification and in the claims section thatfollows, the term “soluble silver compound”, and the like, refers to asilver compound having a solubility of at least 10 grams/liter in wateror in ethanol at 25° C. Preferably, the soluble silver compound has asolubility of at least 25 grams/liter in water or in ethanol at 25° C.,and more preferably, a solubility of at least 50 grams/liter in water orin ethanol at 25° C.

As used herein in the specification and in the claims section thatfollows, the term “volatile solvent”, such as an organic volatilesolvent, refers to a solvent that, in pure form, has a boiling pointbelow 105° C., and typically 100° C. or less, at atmospheric or ambientpressure.

As used herein in the specification and in the claims section thatfollows, the term “non-volatile solvent”, such as an organicnon-volatile solvent, refers to a solvent that, in pure form, has aboiling point above 105° C., and typically above 110° C., at atmosphericor ambient pressure.

As used herein in the specification and in the claims section thatfollows, the term “single-crystal” “monocrystal”, and the like, withrespect to silver particles, refers to a single-crystal silver particleas determined by the standard Electron Back Scattered Diffraction (EBSD)method described in Example 13 hereinabove. Any quantitative assessmentof a fraction or percentage of single-crystal particles within a sample,as used herein in the specification and in the claims section thatfollows, may be performed according to the quantitative EBSDdetermination method described in Example 14. While in theory, thisquantitative EBSD scanning method provides a quantitative evaluation ofa top layer or cross-section of the sample, in practice, thisquantitative evaluation closely reflects the fraction of silverparticles having monocrystalline character, particularly for samplesthat do not have an extremely broad particle size distribution.

Thus, as used herein in the specification and in the claims section thatfollows, the term “majority”, with respect to silver particles, refersto at least one of the following: at least 30% of the randomly-selectedEBSD scans produce a substantially perfect match for a silver singlecrystal, according to the procedure described in Example 14, or morethan 50% of the silver particles, based on the number of silverparticles.

As used herein in the specification and in the claims section thatfollows, the term “standard sintering” or “standard thermal sintering”refers to the sintering procedure described in Example 16.

It will be appreciated that certain features of the invention, whichare, for clarity, described in the context of separate embodiments, mayalso be provided in combination in a single embodiment. Conversely,various features of the invention, which are, for brevity, described inthe context of a single embodiment, may also be provided separately orin any suitable sub-combination. Similarly, the content of a claimdepending from one or more particular claims may generally depend fromthe other, unspecified claims, or be combined with the content thereof,absent any specific, manifest incompatibility therebetween.

Although the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art.Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims. All publications, patents and patentapplications mentioned in this specification, including U.S. Pat. No.6,277,169 and WO Patent Publication No. 2003/080231, are hereinincorporated in their entirety by reference into the specification, tothe same extent as if each individual publication, patent or patentapplication was specifically and individually indicated to beincorporated herein by reference. In addition, citation oridentification of any reference in this application shall not beconstrued as an admission that such reference is available as prior artto the present invention.

What is claimed is:
 1. A method for producing a dispersion of nanometricsilver particles, the method comprising the steps of: (a) reacting atleast one soluble silver compound with an alkali metal hydroxide in anaqueous medium, in a presence of a first dispersant, to produce silveroxide solids having an average secondary particle size below 1200nanometers; (b) reacting said silver oxide solids with at least onereducing agent in an aqueous medium, in a presence of a seconddispersant, to produce a first dispersion of silver particles, thesilver particles having an average secondary particle size below 300nanometers; and (c) removing at least a portion of said aqueous mediumof step (b) from the silver particles to produce the dispersion of thenanometric silver particles; wherein said alkali metal hydroxide andsaid at least one soluble silver compound react in a stoichiometricratio of said alkali metal hydroxide to said at least one soluble silvercompound, and wherein quantities of said alkali metal hydroxide and saidat least one soluble silver compound are added to said aqueous medium ina particular ratio that is at most 0.98 moles of said alkali metalhydroxide to one mole of said at least one soluble silver compound;wherein a concentration of said silver particles within said firstdispersion is within a range of 0.5% to 5%, by weight of said firstdispersion; wherein a concentration of said nanometric silver particleswithin the dispersion of the nanometric silver particles is within arange of 35% to 75%, by weight; and wherein more than 50% by weight ofthe nanometric silver particles are single-crystal silver particles. 2.The method of claim 1, further comprising the step of furtherconcentrating the silver particles within the dispersion of thenanometric silver particles produced in step (c), to form a seconddispersion.
 3. The method of claim 1, wherein step (c) includes washingand concentrating the silver particles, whereby the aqueous medium ispartially removed from the silver particles, to form a concentrate, theconcentrate containing most of the silver particles that were present insaid first dispersion.
 4. The method of claim 1, further comprising thestep of replacing most of said aqueous medium by at least one volatileorganic solvent.
 5. The method of claim 1, wherein said concentration ofsaid nanometric silver particles within the dispersion of the nanometricsilver particles is within a range of 45% to 75%, by weight.
 6. Themethod of claim 1, wherein a concentration of said second dispersantwithin the dispersion of the nanometric silver particles is within arange of 0.2% to 30% of said concentration of said nanometric silverparticles, by weight.
 7. The method of claim 6, wherein saidconcentration of said second dispersant within the dispersion of thenanometric silver particles is at most 5% of said concentration of saidnanometric silver particles, by weight.
 8. The method of claim 1,wherein, subsequent to step (c), the dispersion of the nanometric silverparticles is aged for 6 months, and wherein more than 50% by weight ofthe nanometric silver particles remain single-crystal silver particles.9. The method of claim 8, wherein said second dispersant is, orincludes, said first dispersant.
 10. The method of claim 1, wherein saidfirst and second dispersants are selected from the group of dispersantsconsisting of a polyvinylpyrrolidone (PVP), gum arabic, polyvinylalcohol (PVA), polyacrylic acid (PAA), polyallylamine (PAAm), polysodiumstyrene sulfonate (PSS), 3-(aminopropyl)trimethoxysilane (APS), a fattyacid, lauryl amine, cetyltrimethylammonium bromide (CTAB), andtetraoctylammonium bromide (TOAB).
 11. The method of claim 10, whereinsaid reducing agent is selected from the group consisting of peroxidesand sodium borohydride.
 12. A method for producing a dispersion ofnanometric silver particles, the method comprising the steps of: (a)reacting at least one soluble silver compound with an alkali metalhydroxide in an aqueous medium, in a presence of a first dispersant, toproduce silver oxide solids having an average secondary particle sizebelow 1200 nanometers; (b) reacting said silver oxide solids with atleast one reducing agent in an aqueous medium, in a presence of a seconddispersant, to produce a first dispersion of silver particles, thesilver particles having an average secondary particle size below 300nanometers; and (c) removing at least a portion of said aqueous mediumof step (b) from the silver particles to produce the dispersion of thenanometric silver particles; wherein said alkali metal hydroxide andsaid at least one soluble silver compound react in a stoichiometricratio of said alkali metal hydroxide to said at least one soluble silvercompound, and wherein quantities of said alkali metal hydroxide and saidat least one soluble silver compound are added to said aqueous medium ina particular ratio that is at most 0.98 moles of said alkali metalhydroxide to one mole of said at least one soluble silver compound;wherein a concentration of the silver particles within said firstdispersion is within a range of 0.5% to 5%, by weight of said firstdispersion; and wherein more than 50% by weight of the nanometric silverparticles are single-crystal silver particles.
 13. The method of claim12, wherein said particular ratio is within a range of 0.80 to 0.98moles of said alkali metal hydroxide to one mole of said at least onesoluble silver compound.
 14. The method of claim 12, wherein saidparticular ratio is within a range of 0.80 to 0.96 moles of said alkalimetal hydroxide to one mole of said at least one soluble silvercompound.
 15. A method for producing a dispersion of nanometric silverparticles, the method comprising the steps of: (a) reacting at least onesoluble silver compound with an alkali metal hydroxide in an aqueousmedium, in a presence of a first dispersant, to produce silver oxidesolids having an average secondary particle size below 1200 nanometers;(b) reacting said silver oxide solids with at least one reducing agent,in an aqueous medium, in a presence of a second dispersant, to produce afirst dispersion of silver particles, the silver particles having anaverage secondary particle size below 120 nanometers; wherein saidalkali metal hydroxide and said at least one soluble silver compoundreact in a stoichiometric ration of said alkali metal hydroxide to saidat least one soluble silver compound, and wherein quantities of saidalkali metal hydroxide and said at least one soluble silver compound areadded to said aqueous medium in a particular ratio that is at most 0.98moles of said alkali metal hydroxide to one mole of said at least onesoluble silver compound, and (c) removing at least a portion of saidaqueous medium of step (b) from the silver particles to produce thedispersion of the nanometric silver particles; wherein a concentrationof the nanometric silver particles within the dispersion is within arange of 35% to 75%, by weight; and wherein a concentration of thesilver particles within said first dispersion is within a range of 0.5%to 5%, by weight of said first dispersion.
 16. The method of claim 15,wherein said concentration of the silver particles within said firstdispersion is at least 1%, by weight.
 17. The method of claim 15,wherein said removing at least a portion of said aqueous mediumincludes: replacing at least 50% by volume of said aqueous medium by atleast one volatile organic solvent; followed by (ii) replacing at least50% by volume of said at least one volatile organic solvent by at leastone non-volatile organic solvent.
 18. The method of claim 15, whereinsaid concentration of the nanometric silver particles within thedispersion is within a range of 45% to 75%, by weight.
 19. The method ofclaim 15, wherein said particular ratio is within a range of 0.80 to0.98 moles of said alkali metal hydroxide to one mole of said at leastone soluble silver compound.
 20. The method of claim 15, wherein saidparticular ratio is within a range of 0.80 to 0.96 moles of said alkalimetal hydroxide to one mole of said at least one soluble silvercompound.